So what DO roommates actually do?

Yes, they can age and die, they eat of course and sleep. They flirt sometimes and can serenade your sims and other roommates, but they don't woohoo (unless you have a certain adult mod for that). If they get pregnant, they move out, but you can date them. Not sure about marriage (but I think it's possible).
In my experience they use objects based on their traits (I had an active roommate who was constantly playing basketball), and it's actually better to have some hobby/skill-building items, otherwise they can get bored and move out. They can also move out because their needs are too low, and sometimes there is nothing wrong with the house, roommates are just acting stupid and refusing to take care of themselves.
They do cook, but they have an obsession with white cakes I think... At least it was a thing a year ago. And they can bring food from nowhere.
Actually, roommate sims have a certain bunch of hidden traits that determine their behavior.

ROOMMATE
trait_RoommateNPC_Archetype_Breaker
trait_RoommateNPC_Archetype_Cheerleader
trait_RoommateNPC_Archetype_ClingySocialite
trait_RoommateNPC_Archetype_CouchPotato
trait_RoommateNPC_Archetype_EmoLoner
trait_RoommateNPC_Archetype_Fixer
trait_RoommateNPC_Archetype_LoudMusic
trait_RoommateNPC_Archetype_Mealmaker
trait_RoommateNPC_Archetype_PartyPlanner
trait_RoommateNPC_Archetype_SuperNeat
trait_RoommateNPC_Interest_Art
trait_RoommateNPC_Interest_Baking
trait_RoommateNPC_Interest_Computers
trait_RoommateNPC_Interest_Fitness
trait_RoommateNPC_Interest_Music
trait_RoommateNPC_Quirk_Absent
trait_RoommateNPC_Quirk_BathroomHog
trait_RoommateNPC_Quirk_BigCloset
trait_RoommateNPC_Quirk_LateOnRent
trait_RoommateNPC_Quirk_Prankster
trait_RoommateNPC_Quirk_PublicAffectionDisplayer
trait_RoommateNPC_Standard

https://forums.thesims.com/en_us/discussion/comment/17312724/#Comment_17312724

As for mods, I can recommend littlemssam's Choose Your Roommate, it doesn't change their behavior, but makes a process of choosing one easier. Also it can increase a roommate cap to 20 I think.

My roommate kept baking white cakes….. lol.

Err, I have a character who dated two of her roommates and WooHoo was definitely an option. The first roommate moved out randomly afterward and had the noncommittal trait so I just said that it didn't work out. The second one is now playable in a romantic relationship with that sim. It is good to know that randomly moving out may just mean that the sim isn't meeting their needs. You also interview roommates and decide who stays.

I just consider it a feature. I haven't used it that much but it is good to have there in case you want that. They pay rent as well. If you have a spare room with a bed, you can add a roommate. I only use it with specific households. I have also made my own playable roommates which I've sometimes done if someone was having a hard time finding love.

Pay rent, complain about not having their needs met (easy fix for it though), invite friends over and bake white cake... lots of white cake.

They also mention about leaving space around their bed to place their stuff down, but I've never played with them outside of university housing/dorms so not sure how this plays out in residential lots.

One of my room mates used to leave passive aggressive notes behind in her room about me not visiting her room while she was away. She used to serenade my sim a lot but my sim had no time for such shenanigans. Rent is rent and my sim was not swinging that way anyway. :lol: She eventually left because her needs were not met. Now the house is bigger and there are three new room mates. I can tell the male is into fitness, piano playing, and is a slob, he leaves dirty plates everywhere. I can direct them to help with cleaning and the ladies do help out. No white cakes so far, but one of them did cook dinner once.

"EmmaVane;c-17916722" wrote:
They also mention about leaving space around their bed to place their stuff down, but I've never played with them outside of university housing/dorms so not sure how this plays out in residential lots.

It plays even better in residential lots. When my sim was in Uni, he didn't love or hate his roommates but the family mentioned below is a completely different story. I've also had many roommates in many households in many saves and enjoyed them much more than the ones at Uni.

"crocobaura;c-17916740" wrote:
One of my room mates used to leave passive aggressive notes behind in her room about me not visiting her room while she was away. She used to serenade my sim a lot but my sim had no time for such shenanigans. Rent is rent and my sim was not swinging that way anyway. :lol: She eventually left because her needs were not met. Now the house is bigger and there are three new room mates. I can tell the male is into fitness, piano playing, and is a slob, he leaves dirty plates everywhere. I can direct them to help with cleaning and the ladies do help out. No white cakes so far, but one of them did cook dinner once.

My roommate in the family I'm playing now is the father of the toddler twins of my main sim (probably why my main sim's wife left him), things got a bit out of control. Anyway, I saw him one time at the climbing wall but never saw him climb it. I've found numerous white cakes started be few actually completed. I usually have my sim just throw them all away since we don't get that "wasting food" moodlet in Sims 4.

He's never left a passive aggressive note (other roommates in other households loved to leave them) but he has twice said that his needs were not being met. The first time was after I assigned him to my main sim's bed when the wife left with the father of her second child. When he complained, I assigned him back to the bottom bunk in my sim's eldest son's room (an alien child from abduction) and he was happy again.

Later, he complained again that his needs weren't being met (by the way, he's a merman and we live in Willow Creek so yeah talk about needs not being met). I assigned him to the bottom bunk in my sim's daughter's (the only child he shares with his wife) room. I immediately reassigned him to the bottom bunk in the boy's room and his needs were once again met.

So it appears to me that if they complain about their needs, play a bit of musical beds and they'll reset.

They just exist as any other sim would if you visited their house basically. Sometimes they'll help out with housework and do useful things, other times they won't.

"ChampandGirlie;c-17916694" wrote:
Err, I have a character who dated two of her roommates and WooHoo was definitely an option.

Yes, woohoo is an option of course, they just won't woohoo on their own (I don't think). Maybe if you make some of your roommates couples they will woohoo autonomously, I'm not sure. I rarely see autonomous woohoo in the game.

"somewhsome;c-17916776" wrote:

"ChampandGirlie;c-17916694" wrote:
Err, I have a character who dated two of her roommates and WooHoo was definitely an option.

Yes, woohoo is an option of course, they just won't woohoo on their own (I don't think). Maybe if you make some of your roommates couples they will woohoo autonomously, I'm not sure. I rarely see autonomous woohoo in the game.

have you tried "talking up sim" between the two to see if that encourages them to visit the local dumpster? How romantic?!?!